Studies in which rat peritoneal mast cells were separated by elutriation into successive fractions of increasing cell size and maturity revealed that immature mast cells (Stages I and II) were found to have high histamine synthetic activity and low (less than 1 pg/cell) histamine content. With cells of increasing size and maturity synthetic activity decreased 90% and histamine content increased to 12 pg histamine/cell as did the number of intracellular granules. A gradation in response to compound 48/80 was also observed: small mast cells (Stage I) were resistant, those of larger size (Stage II) were partially responsive and large mast cells (Stages III and IV) were fully responsive to compound 48/80. Although mature mast cells released 68-83% of their granular histamine content in response to compound 48/80, intracellular histamine synthetic activity was unchanged. From our biochemical and histological studies, we conclude that during mast cell maturation there is a progressive increase in granule numbers and granule constituents (heparin and histamine), and increase in responsiveness of the cells to compound 48/80 and a decline in extragranular histamine synthetic activity.